Mar 31 2009

Microdynamos and the Piezoelectric Effect

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In our increasingly electrified world the race is on to find new sources of energy and batteries to store that energy. Applications range from the very small to the huge. The electric car industry is essentially waiting on improvements in battery technology.

Researcher Z. L. Wang from the Georgia Institute for Technology has recently announced a breakthrough at the other end of the size spectrum – the very tiny. He has created, not a battery, but a nanoscale energy generator that uses the piezoelectric effect to convert movement into electrical current. The effects are still modest – 0.2 volts with an efficiency of 6.8%, but this is enough for some applications.

The piezoelectric effect is a property of some materials that converts mechanical energy into electrical current. Our bones have this property and it is that which causes the molding of bones under pressure (such as moving the placement of teeth in the jaw using braces).  The amount of current generated is generally small – we won’ be running our cars off the piezoelectric effect, but in the aggregate can be useful.

What Wang has done is create rods with nanoscale zinc oxide bristles. When jostled the bristles move against each other, bending them and generating current through the peizoelectric effect. The innovation here is the extremely small size. While the current generated is still small, if enough of these microdynamos were put together they could be useful for some applications. But most applications will have to wait until improvements are made and higher efficiencies are achieved.

Potentially such nanoscale piezoelectric devices could be used to research all of the small electronic devices that we carry around with use – cell phones, mp3 players, beepers, watches, cameras, etc. Just from walking around we generate and waste a great deal of mechanical energy. Some of that energy can be recaptured and put to use.

One exciting potential application is in implantable medical devices. One major technical limitation of such devices is that they need energy, but once inside the body it’s hard to replace the batteries. Some devices, like even the best current artificial hearts, require external batteries. Others, like pacemakers, can be charged from the outside using a coil or need to be removed to have their batteries changed every 7-10 years.

But what if such devices could be charged by the movement of the body itself – by biological energy. Pacemakers, for example, could be charged by the energy of the beating heart. (Such systems are already in development using microgenerators.) The constant expansion and contraction of the chest and diaphragm for breathing is another source of mechanical energy.

And, of course, walking can be a significant source of mechanical energy. Every time our feet strike the ground the impact dissipated mechanical energy throughout our shoes, feet, legs, and hips. Every step could be used to harvest some of that energy. Swinging arms could also be exploited for energy.

Also – deliberate activity could be employed. If your cell phone is running a bit low, just shake it for a few minutes.

Biological activity is not the only source of ambient mechanical energy. Our world is full ove movement and much of the energy of that movement is dissipated uselessly to the environment. For example, the energy of rain striking the ground is completely wasted.  If, however, it struck a piezoelectric plate the energy of the falling droplets could be harvested.

Energy is the ultimate currency of our civilization. As we search for greater efficiency of energy production and use the ability to extract ambient energy from the environment will be extremely useful. The piezoelectric effect allows us to connect mechanical energy to electricity, and this property can be exploited in numerous ways, limited only by our cleverness. This is definitely a technology to watch.

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